If immery was here she would give you a better answer, but basically, Polish doesn't care much about this/that distinction as long as you don't have to show the contrast between them (This dog is black and that one is white). So in most cases, both "this" and "that" are accepted here.
Whether Polish doesn't care about the difference between "this" and "that", I am going to translate "tego" as "this" and "tamtego" as "that". I just think it is easier that way since that is the correct translation for each word. Plus, we beginners don't need to be confused even more when Polish is already a very complicated language.
"This" (ten) has a bunch of different forms in Polish, depending on gender, number, case and animacy. Since "chłopca" is singular masculine animate accusative, you must use the appropriate form of the demonstrative pronoun "ten". Take a look at this table to see all forms:
The audio may sound wrong (the intonation is often wrong) but you still see the question mark when translating, don't you?
Still, your answer should have been accepted, it's on the list of correct ones. If a Polish question doesn't start with "czy", we allow to interpret it as a surprised "What? You know this boy?!", so it works.